Now.
Previously, before cider was used, we had class full address masks, so we had class addresses, class
B addresses and class C addresses.
A Class A subnet supports about 16 million addresses and uses the mosque.
255.0.0.0.
Class P networks support about 65,000 host addresses with a mask of 255.255.00.
Class C Networks support 254 host addresses and use a mask of 255225 5 to 55.0.
The problem with this method is what happens if a company wants to support 3000 hosts, for example?
So if the network had 3000 hosts, which of these three address classes with the company get?
Well, they could get a class B address, but that's an awful waste of a lot of addresses.
About 65,000 addresses are available in a Class B subnet.
But in this example, the company only needs 3000 host addresses.
They could get multiple class C networks, but that means that they're going to be allocated many Class
C addresses, which has a negative effect on Internet routing tables.
Now, rather than doing that, we can move the subnet mask insider notice in Class A, B and C addresses.
The subnet mask is set on the octet boundary notice in class A, B and C addresses the subnet mask is
set on the octet boundary plus a first eight bits.
Class B First 16 bits.
Class C first 24 bits.
But rather than doing that with CIDR, the subnet mask can be somewhere in the middle.
It doesn't have to be on the octet boundary.
Here's another example.
In this example, we've got a subnet mask of 255.224.0.0245.
In binary is eight binary ones.
2 to 4 is three binary ones, followed by five binary zeros.
So if we count the number of binary ones, that gives us 11.
So 250 52240.0 is the same as slash 11.
The network portion is on the left hand side, the host portion is on the right hand side.
But notice once again that the divider between network and host is not on the octet boundary, it's
somewhere within the octet.
So in this example, the most significant three bits of that octet are network and the remaining portion
of the octet is host.
Now in the subnet videos, I'm going to show you this in a lot more detail.
We're going to work out various types of subnets.
What I need you to understand with regards to CIDR at this point is that CIDR allows us to implement
a variable length subnet masks.
No longer do we have class A, B and C networks where class A is always slash eight, class B is always
slash 16, and class C is always slash 24.
When CIDR is used, the mosque's can vary and that's what we use today.
So from 1993, CIDR is more preferable than class full network masks.
Now, once again in the subheading videos, I'm going to discuss subnets in a lot more detail, so please
refer to those videos if you want to learn more about how to subnet a network or how to work out the
maximum hosts that can be supported on a subnet, or which addresses the first address or last address,
or what is the broadcast address and so forth for a specific subnet.